Examples of what are known in digital X-ray imaging are image intensifier camera systems based on television or CCD cameras, storage film systems with an integrated or external readout unit, systems with optical coupling of the converter foil to CCD cameras or CMOS chips, selenium-based detectors with electrostatic readout, and solid state detectors with active readout matrices with direct or indirect conversion of the X radiation.
In particular, for a few years solid state detectors have been finding application for digital X-ray imaging. Such a detector is based on an active readout matrix, for example made from amorphous silicon (a-Si), that is precoated with an X-ray converter layer or simulator layer, for example made from cesium iodide (CsI). The incident X radiation is firstly converted into visible light in the scintillator layer. The active matrix is subdivided into a multiplicity of pixel readout units with photodiodes that in turn convert this light into electric charge and store it in a spatially resolved fashion.
An active readout matrix made from active silicon is likewise used in the case of a so called directly converting solid state detector. However, this active readout matrix is arranged downstream of a converter layer, for example made from selenium, in which the incident X radiation is corrected directly into electric charge. This charge is then stored, in turn, in a pixel readout unit of the readout matrix. Reference may also be made as regards the technical background of a solid state detector to M. Spahn et al., “Flachbilddetektoren in der Röntgendiagnostik” [“Flat image detectors in X-ray diagnostics”], Der Radiologe 43 (2003), pages 340 to 350.
The amount of charge stored in a pixel readout unit determines the brightness of a pixel of the X-ray image. Each pixel readout unit of the readout matrix therefore corresponds to a pixel of the X-ray image.
A characteristic of an X-ray detector that is decisive for the image quality is that the detector efficiency of the individual pixel readout units deviate from one another more or less strongly. This can be seen in that two pixel readout units supply raw image values with different brightness even if they are irradiated with the same light intensity. The resulting, unprocessed raw X-ray image has a comparatively poor image quality because of this brightness fluctuation. Other factors contributing to the brightness fluctuation are spatially dependent fluctuations in the thickness of the scintillator layer, the dependence of the scintillator layer on the radiation quality, a qualitatively different adhesion between scintillator layer and detector plate, and inhomogeneities in the irradiated X-ray field. Consequently, a continually strong brightness drop usually occurs in the edge regions of the X-ray detector.
It is customary to calibrate a digital X-ray detector in order to improve the image quality. To this end, a calibration image is recorded in conjunction with a constant X-ray exposure, which is also denoted as “gain image”. The brightnesses of the individual pixels, the gain values, are compared with a normalized gain brightness value. If the measured gain value is above a defined upper threshold value (for example 200 percent), or a below a defined lower threshold value (for example 25 percent) of a gain value normalized over all the pixels, the corresponding pixel readout units are assumed to be defective and either no longer used at all for the X-ray imaging, or subjected to a complicated correction of defects. The remaining gain image is mathematically combined with the X-ray images recorded during the later normal operation of the X-ray detector, for example multiplied such that the brightness fluctuations present in both images in about the same way are at least partially compensated.